The lead arm should stay as close to how it is in normal sprinting motion as possible. The hand of the lead arm should never cross the nose of the face, and should never raise any higher than the forehead. The elbow should open up just wide enough for the trail leg to drive through in its upward motion. The elbow should never be in a position where it is higher than the hand. The lead arm should begin its downward motion before the lead foot reaches the crossbar. The elbow should remain bent as you drive the lead arm back down. Hand should return to the back pocket as you return to sprinting.
I have been placing a lot of efforts into teaching the lead arm mechanics of one of the athletes I am coaching that is a junior in high school. His lead arm needs to initiate attack. I say attack the hurdles as self preservation overrides many coaching that is done in the event. For me I see the lead arm piercing the heart of the left side of the enemy while cutting away diagonally before bringing back the elbow. The above woodcut photo is the Gran Simularcro or dynamic lunge of fighting and reminds me of the delayed take off of the 110 hurdles. The l’incredibile accrescimento della botta lunga in Capo Ferro’s manual is a committed tactic and gives up the defense of the attacker and is a great way to teach the take off by forcing them to choose to be aggressive. The manuals I have read really have helped me see the music that Coach Wilbur Ross shares in the hurdlers bible as rhythm is all about tempo. The writers of the ancient manuals were true renaissance men and created works that gave the reader a sound understanding of how the art of the rapier really worked. Their complete background of art, mathematics, science, and music gave them a huge perspective that could connect all of the vital patterns found in the art of fencing. Reading those materials have made my approach to the hurdles more aggressive. The scanned image is very interesting as you can see the faint outline of the geometric angles and you can google the images of clearer photos. Early biomechanics?